Secret Codes #6
Remember Caesar cipher (way back in Secret Codes #3)? That cipher can be broken just be trying all possible combinations. And since there are only 26 possible encodings, it's not very secure. And we could also break a Caesar cipher by looking at frequency analysis (as in Secret Codes #4).
But what if every letter of a message had its own Caesar cipher? So 2 letters that are the same in the original come out as different letters in the ciphertext. Now there are... well, a LOT more possible encodings and it's getting a lot harder to crack the code!
Here's an example of how to use this technique called the Vigenère cipher. The message you want to encode (called the "plaintext") is "Burn all documents". The key is "watermelon". Since the key is shorter than the original, just repeat the letters to make it the same length as the plaintext. Cross reference the first letter in the plaintext ("B") with the first letter in the key ("W") and that gives the first letter in the ciphertext ("X"). See the graph below.
Decipher in the reverse order. If you know the key, use the column of each letter in the key to find the letter in the ciphertext. The row of that letter is the plaintext letter.
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